Bevel is not symmetrical
It seems your object has non-uniform scale.  Apply the scale of the object with Ctrl+A > scale to fix this problem.

Beveling with non-uniform scale:


Applying the scale:


Beveling after applying scale:


When you scale an object in object mode, it is not really "applied" to the object's geometry.  I like to think of scaling as a modifier that is always last in the stack, whatever you do in edit mode (or most other places) is as if it was done before the object was scaled.

Check out this question for more uses of applying scale.